A plasmon is the quantization of plasma oscillations, which are density waves of the charge carriers in a conducting medium such as a metal, semiconductor, or plasma. Surface plasmons exist in various geometries, such as nanoparticles or two dimensional films. Thin film plasmons propagate in the micron range depending on the wavelength and type of material. Such plasmons are non-radiative in air and are sensitive to dielectric environment. Recent technological advances that allow metals to be structured and characterized on the nanometer scale triggered new interest in the application of surface plasmons (SPs). The control of SP properties is of interest to a wide spectrum of scientists, ranging from physicists, chemists and materials scientists to biologists. For instance, SPs are being explored for their potential in small-scale optical circuitry, high-resolution optical microscopy and bio-detection.
Surface plasmons are known solutions of Maxwell's equations applied along an interface between a medium with a negative permittivity, i.e. a metal, and a dielectric. These solutions are traveling waves that are generally bound to the interface and are exponentially decaying in both media. The optical excitation of surface plasmons on flat metal interfaces is challenged by the phase matching condition between the plasmons and the exciting radiation. The surface plasmon dispersion ω(k) is located outside the light cone ω=ck and hence no SPS can be excited with freely propagating radiation. The excitation of surface plasmons can only occur if the photon momentum—or the wave vector—can be artificially increased. Various experimental techniques have been developed to accomplish this task, such as (i) increasing the index of refraction of the incident medium (total internal reflection (TIR) conditions) or (ii) engineering the surface of the film (grating coupler). While these approaches provide very efficient coupling between the incident photons and the SP waves, the interaction area is usually comparable or greater than the SP propagation distances.
It was recognized very early that in an asymmetric structure, i.e. a thin metal film (permittivity εm) surrounded by two dielectric media (permittivities ε1, and ε2, with ε1>ε2), has four modes that are solutions of the dispersion relations. Two of these solutions exist at each of the interfaces εm/εi, i=1, 2) and are characterized by their fields decaying exponentially into the media. The two other modes are radiative leaky waves originating from the finite thickness of the film. As a non-radiative mode travels along an interface, the wave amplitude decays exponentially in the metal and is coupled into leakage radiation (“LR”) by the opposite interface. The far-field observation of this leakage radiation (LR) gives a direct measurement of the non-radiative surface plasmon propagation at the opposite interface. The intensity of the radiation, at a given lateral position in the film, is proportional to that of the SP—at the same position.
Surface plasmons are thus well-known phenomena and commercial surface plasmon-based sensors are currently used in biological research and in industrial applications. Use of the surface plasmons allows manipulation of light in devices smaller than the wavelength and can be extremely localized. They also exhibit ultrafast dynamics for use in rapidly changing circumstances or for rapid data output. For example, the detection principle of a commercially available plasmon sensor relies on the surface plasmon resonance resulting from energy and momentum being transformed from incident photons into surface plasmons. This process is sensitive to the refractive index of the medium on the opposite side of the film from the reflected light. Heretofore, the light source used for optically exciting surface plasmons was a monochromatic laser directed at an angle through a prism to a metal (gold or silver) coated surface. The sensor operated by determining the variation in the angle of incidence for maximum plasmon absorption. The presence of an adsorbate material on the surface of the metal was detected by measuring the change in the angle of incidence of the monochromatic beam. Alternatively, the angle of incidence was fixed and the wavelength varied to extract the same information. These methods are, however, very time consuming and technically more difficult if one wishes to extract spectral information, e.g. multiple wavelengths, on the adsorbates material.
Further, in the vast majority of other surface plasmon studies, they are optically excited in the so-called Kretschmann attenuated total internal reflection (“ATR”) configuration, where the momentum mismatch between free-propagating photons and SPs is taken from a material with a refractive index larger than air, e.g. a glass substrate. In the case of an ATR geometry and for smooth metal films, the leakage radiation (“LR”) interferes destructively with the incoming excitation light at the reflection spot and cannot be detected if the excitation area is larger or comparable to the lateral decay length of the surface plasmon. However, if surface plasmons are locally excited by electrons or near-field techniques, LR can be observed. This is, however, technologically ambitious and difficult because near-field optics is based on scanning probe microscopy.